Heretofore the preparation of sero-sanguineous body cavity fluid specimens for cytologic microscopic evaluation has usually been a time consuming process and has frequently given rise to poor cell quality, thereby hindering complete cell evaluations. In a large part, these difficulties have resulted from the equipment used.
Present preparation techniques can take 45 minutes or longer to complete. In cases in which sparsely populated cytologic specimens are utilized, the process requires the processing of relatively large specimen samples, and multiple pieces of equipment with only a few cells being collected in each run. This requires the added and potentially cell distorting step of combining the various cell samples into one sample for evaluation. The present standard collection techniques are filtration and flotation processes and centrifugation methods.
The flotation process, which is a sedimentation process, is described in "A Manual of Cytotechnology ASCP (1977), pages 310-311. In the flotation process the cell, in its original solution or in a physiologic saline solution, is carefully layered over an albumin solution, having a specific gravity of 1.050 to 1.060, in a centrifuge tube. The tube is then centrifuged causing a separation of the cells, said separation being dependent upon the cell's specific gravity and the specific gravity of the albumin solution utilized. The overall process is time consuming and frequently results in poor cell recovery. The process is also limited by the size of the centrifuge tube leading, in many cases, to the use of multiple centrifuge tubes, and concomitantly many individual processes. The cells from the individual tubes must then be collected into one cell sample.
In the conventional centrifugation process, the body fluid, such as blood, is placed in an elongated tube, such as a test tube or a specifically designed centrifuge tube, and the tube subjected to a centrifugal force to separate the cells from the serum fraction. In this process the cells to be collected are forced to the bottom of the tube. The supernatant liquid is then removed and the cells resuspended. Utilizing a pipette, one to two drops of the cell suspension is then placed in a tube for the cytospin (cytocentrifuge) which is then centrifuged while the cells concomitantly are automatically placed on a microscopic slide in a compact circle. An absorbent paper against slide absorbs the liquid from the suspension. Unfortunately the absorbent will also absorb some of the cells thus decreasing the already small amount of cells which is available for deposition on the microscopic slide.
The cytospin can also be utilized in conjunction with the flotation process.
For the filtration process, there are numerous apparatuses and membrane filter materials on the market which perform adequately for the collection of cells. Unfortunately there are many pitfalls in the use of membrane filters and the collected cells, in many instances, are damaged and hence of little value for microscopic examinations.
Prior to the introduction of membrane filters, sparsely populated cytologic specimens contained so few cells that it was physically impossible to recover them and make cell spreads. This difficulty was essentially eliminated by the use of membrane filters. Membrane filters are in general made from cellulosic products or polycarbonates.
In the filtration process, sparsely populated specimens can be filtered directly but other more populated specimens should be centrifuged first, the supernatant removed and the cell concentrate resuspended. In this process the filtration is a vacuum filtration process which packs the cells against the filter thus possibly damaging the cells. Also, if the technician is inexperienced or insufficient attention is being paid to the process, it is possible for the system to be pulled dry, i.e., most, if not all, fluid is removed from the cells and filter thus drying the cells. This too will result in damaged cells.
Ideally, cells on a filter from the filtration process, should be randomly distributed on a monolayer, without overlapping.
The present invention involves an apparatus which combines the beneficial aspects of the flotation and filtration processes but eliminates the disadvantages associated with each process. Most importantly, the present invention provides a means for obtaining cells in an essentially monolayer arrangement and with essentially no distortion.